1. Field of the Invention
This invention relates generally to fluid purification systems, and more particularly to gas purification systems used to provide highly purified gasses for use in semiconductor manufacturing equipment.
2. Description of the Related Art
Fluid purification systems are used to remove impurities from contaminated or otherwise impure fluids. The fluids may be liquid and/or gaseous, and are typically purified to provide a high quality fluid source for manufacturing or consumption purposes, or to remove toxins and other contaminants prior to the disposal of the fluid. Such systems usually include "consumables" such as filters or getters, which must be periodically replaced.
In the semiconductor manufacturing industry, gas purifiers are used to provide the highly purified gasses used in the semiconductor manufacturing process. For example, argon (Ar) must have less than 10 parts per billion (ppb) of impurities if it is to be used in-state-of-the-art semiconductor manufacturing equipment. Argon available from commercial gas suppliers typically has about 500 ppb of impurities, including water (H.sub.2 O), oxygen (O.sub.2), nitrogen (N.sub.2), carbon monoxide (CO) and carbon dioxide (CO.sub.2 2). Gas purifiers are used to purify the commercially available gas to impurity levels acceptable to the semiconductor manufacturer.
It is apparent from the above example that the terms "pure" and "impure" are relative, not absolute, terms. For example, commercially available argon gas having 500 ppb of impurities can hardly be considered heavily contaminated. Therefore, as used herein, "pure" gasses will be defined as gasses which meet the purity specifications for a given process, and "impure" gasses will be defined as those gasses which do not meet those purity specifications.
There are several methods for removing impurities from gasses such as argon. For example, Semi-Gas of San Jose, Calif. produces a resinous filter under the trademark NANOCHEM which removes impurities from gasses. While resinous filter systems are fairly effective in removing certain impurities from gasses, they sometimes introduce organic contaminants into the gasses being purified.
A preferred type of gas purification system for use in the semiconductor manufacturing industry is a getter-type purification system. Getter materials comprise certain metal alloys which have a chemical affinity for certain gasses. In these systems, getter materials are encased in stainless steel columns and are heated to a temperature in the range of 300.degree.-450.degree. C. Impurities from gasses flowing through a getter column are trapped by the getter materials, thereby providing a purified gas at the outlet of the getter column.
For the most part, the gettering process is not reversible, resulting in the eventual saturation of the getter material with the impurities. Therefore, getter-type materials have a finite "life-time", making the getter columns a "consumable" item. Getter-type gas purification systems are available from SAES Pure Gas of San Luis Obispo, Calif.
FIG. 1 illustrates a commercially available getter-type purification system 10 made by SAES Pure Gas as model MEGATORR. System 10 includes a heated getter column 12, a mass flow meter (MFM) 14, three valves 16, 18, and 20, and end-of-life (EOL) probe 44, and an EOL analyzer 24. An inlet 26 is coupled to a source of impure gas (not shown).
In operation, valves 16 and 18 are opened and the gas to be purified flows through a conduit 28, valve 16, a conduit 30, MFM 14, a conduit 32, getter column 12, a conduit 34, valve 18, a conduit 36 and, finally, out of an outlet 38. The MFM 14 is used to monitor the amount of gas flowing through the system, and the heated getter column 12 removes impurities from the gasses flowing through the system 10.
Valve 20 can be used to bypass the getter column 12. To accomplish the bypass, valves 16 and 18 are closed, and valve 20 is opened. The gas flow is then into inlet 26, through conduit 28, through conduit 40, through valve 20, through conduit 42, through conduit 36 and, finally, out of outlet 38. When the getter column 12 is bypassed in this fashion, there is no purification of the gas flowing through the system 10.
The getter columns 12 contain substantial quantities of expensive getter material and have a finite useful life-time. For example, a 10 m.sup.3 /hr getter column 12 containing 10 kg of getter material lasts about one year at 10 m.sup.3 /hr of gas flow with a 5 parts per million (ppm) inlet impurity concentration and costs about $30,000.00 U.S. The actual period of time that the getter column 12 lasts depends upon the type and amount of impurities, the flow rate of the gas, the duration and frequency of the gas flow, and a number of environmental factors.
Because getter columns tend to be expensive, semiconductor manufacturers want to get as much use out of them as possible before they are replaced. Simply relying on a reading from MFM 14 as to when to replace the getter column will often result in discarding a getter column which may only have consumed a fraction of its allotted lifetime. This is because the MFM 14 measures only total gas flow, and is not able to measure impurity concentration. However, semiconductor manufacturers tend to err on the side of conservatism, since using a getter column past its allotted lifetime can result in inadequate gas purification and a possible contamination of a semiconductor manufacturing process utilizing the impure gas, which can be considerably more costly than the cost of replacing a getter column.
A partial solution to this problem is the use of an end-of-life (EOL) sensor 44 disposed within the getter column 12. The getter pellets 46 within the column 12 have a characteristic resistance which increases as their capacity to remove contaminants reduces. This increasing resistance is primarily a function of the amount of oxygen absorbed by the getter pellets 46, resulting in the formation of high resistance oxides. While the primary limiting factor on the life of a noble gas getter column 12 is its absorption of nitrogen, nitrogen absorption is related to oxygen absorption which, as explained previously, is related to the resistance of the getter pellets 46. The EOL analyzer 24 measures the resistance of the getter pellets 46 between EOL sensor 44 and the conductive walls of the getter column 12 to estimate the remaining life in the getter column 12. EOL sensors and analyzers are provided on argon and nitrogen versions of the aforementioned MEGATORR system sold by SAES Puregas, Inc.
While the resistive EOL system comprising sensor 44 and analyzer 24 is a major improvement in predicting the end-of-life of a getter column, it does have some drawbacks. For one, this resistive EOL system is not entirely accurate, because it can only extrapolate nitrogen absorption (the limiting factor for the column) from an estimate of oxygen absorption. Since the proportion of oxygen to nitrogen in the inlet gas can vary, assumptions made about the relationship to oxygen and nitrogen absorption can be erroneous.
Another drawback is that the installation of the resistive EOL sensor into the getter column 12 adds to the cost of the column and increases its fragility. This is because a high temperature, gas-tight seal (such as a ceramic grommet) must be provided through a wall of the getter column 12 to accommodate a connecting wire 48 extending between the EOL sensor 44 and the EOL analyzer 24. If this seal breaks or begins to leak, the getter column may have to be replaced.
The resistive EOL sensor is furthermore currently restricted for use with certain types of gas purification systems. While the EOL sensor works well for argon and nitrogen getter-type gas purification systems, it is not well adapted for use in getter-type gas purification systems which purify hydrogen (H.sub.2), ammonia (NH.sub.3), or silane (SiH.sub.4). There is therefore a need for a method for predicting end-of-life of a system consumable in a fluid purification system which cannot use a resistive EOL sensor.